Hi I got this issue again with my newly set up Unity Project.
deleting the System.dll Reference and adding it again plus reloading doesn't work.
letting Unity compile also doesn't solve the issue.
OS, Unity version, Rider version?
Check that in Rider full MSBuild is selected. Not a dotnetcore one.

feel free to reopen, if my previous suggestion didn't help
Most helpful comment
Check that in Rider full MSBuild is selected. Not a dotnetcore one.
